The present invention relates to spark erosion machining, and more particularly, to a novel device for machining a gas turbine engine component, such as a low pressure turbine (LPT) shroud or the like, and a novel method for automatically correcting for the continuous wear or erosion of the wheel-shaped electrode of a Spark Erosion Machining (SEM) device during a machining operation.
A SEM device may typically include a computer numerical controlled (CNC) lathe having a faceplate type fixture mounted to a lathe spindle to hold a cylindrically shaped gas turbine engine component or the like for rotation about a cylindrical axis of the component during machining. The spark erosion machining is performed by a rotating metallic wheel-shaped, or disk-shaped, electrode. The wheel-shaped electrode is connected to one terminal of an alternating current power supply, and the oppositely polarized terminal of the power supply is connected to the component or workpiece by a brush contact which rides on an axis of the lathe spindle of the SEM device. Depending upon the A.C. voltage applied between the component and the wheel-shaped electrode, when the electrode is brought within a certain distance from the component to be machined, an electrical arc will jump between the electrode and the oppositely polarized component being machined. This arc or spark will cause a portion of the material of the component to be removed or vaporized. During removal of metal from the component part, material is also eroded from the wheel-shaped electrode; thus, the size of the electrode continually decreases during the machining operation.
Several machining passes or cycles may typically be required to machine or mill a component part, such as an LPT shroud or the like, to design specifications. Unless the dimension of the SEM wheel-shaped electrode is measured before each cycle and the SEM device is adjusted to correct for the variation in electrode size, the component may not be precisely machined to the specified dimension. Many components measured after machining have been found to have more material removed than required by design specifications or not enough material was removed.
To consistently machine an LPT shroud, having an interior portion formed of a multiplicity of honeycomb-shaped structures, to specified dimensions, it is necessary for a machine operator to measure the largest distance or radius between the rotational axis of the wheel-shaped electrode and the outer peripheral edge of the electrode before each machining pass or cycle. The electrode may not wear evenly about its circumference and therefore care should be taken to determine the largest radius each time. The operator will then make adjustments to the SEM device in order to take the proper depth of cut during each successive machining cycle in order to obtain a shroud with the specified dimensions.
This method may reduce many machining errors, depending upon the skill of the operator, but the time to completely machine an LPT shroud is substantially increased. Machining errors may still occur if an operator incorrectly measures the largest radius of the wheel-shaped SEM electrode or incorrectly inputs a correctly measured radius into the computer numerical control (CNC) of the SEM device.